Due to an explosion in both civilian and military wireless communication, there is a growing need for high speed, reliable, secure, wireless communication of large amounts of data between communicating nodes. Traditional communication by wireless radio frequencies suffers from several shortcomings, including rapid attenuation of signal strengths with distance and limited available communication bands. In addition, radio signals can be easily intercepted by unintended and sometimes hostile receivers. Furthermore, it is relatively easy for hostile antagonists to attempt to jam radio communications by transmitting radio signals at high energies that blanket a region of interest.
Laser communication offers an attractive wireless alternative to radio communication, especially when point-to-point communication is required, because the non-dispersed, focused character of laser communication intrinsically avoids most of the problems that are associated with radio communication. Laser communication eliminates the need for frequency planning and authorization, and circumvents the highly congested RF spectrum bandwidth constraints that limit the practical data rates available to users of RF links. Laser signals also experience very little attenuation as a function of distance, because the signal energy remains tightly contained in a beam from a diffraction-limited optical aperture with exceptionally low divergence. Also, laser communication security is intrinsically high, because interception of and interference with laser communications requires direct interception of a laser communication beam, and/or focusing jamming beams directly at an intended signal receiver.
One important application that can benefit significantly from laser communication is satellite communications, where line-of-sight access is generally available, and where the communication distances are very great. Global laser communication can be realized by forming a laser communication network among a plurality of satellites. Laser communication can provide data rate communications for satellites that are much higher than radio data rates, with unmatched anti-jam characteristics and an inherently low risk of communications intercept.
Nevertheless, while much of the present disclosure is presented in the context of satellite laser communication, it will be understood by those of skill in the art that the present disclosure is not limited to satellite communication, but also applies to other implementations of laser communication.
Full duplex (simultaneous, bi-directional) laser communication can be implemented between terminals in a laser communication network to maximize the network connectivity and bandwidth, and minimize handshake overhead. Typically, two laser wavelengths are implemented in the network, whereby each satellite transmits on one of the two wavelengths and receives on the other. The two implemented wavelengths can generally be referred to as a “first” wavelength and a “second” wavelength. However, for simplicity of expression, the two wavelengths that are implemented in a full duplex laser communication network are sometimes referred to herein as “red” and “blue,” although any two wavelengths can be selected, including wavelengths that are not within the visible spectrum. The two wavelengths are typically selected such that the receive channel can be isolated from the transmit channel by a factor of 60 to 100 dB or more.
Typically, each of the terminals that form a laser communication link is either a “red” terminal, in that it is equipped with a “red” message transmitting laser, or it is a “blue” terminal, in that it is equipped with a “blue” message transmitting laser, and the terminals are configured such that “red” satellites are placed into direct communication with “blue” satellites, and vice versa, thereby enabling full duplex communication. Of course, the receivers in the “red” satellites include filters and/or other components that are configured to direct received “blue” laser light to the detector of the receive channel, while excluding any other wavelengths from reaching the detector, and the receivers in the “blue” satellites include similar components that allow only “red” laser light to reach the detector.
Laser communication requires that precision pointing and alignment between the optics of two communicating terminals be established and maintained. This can be challenging, especially when the terminals are separated by a great distance. Generally, one or both terminals transmits an alignment “beacon” that is slightly dispersed in its focus, so that it can be detected by the other terminal. Typically, each beacon is transmitted at a wavelength that is neither the “red” nor the “blue” wavelength. The beacons are used both for initial alignment during an “acquisition phase,” and sometimes also, with reduced power and tighter focus, for maintaining alignment during data communication (“communication phase”). This use of beacons to establish precise alignment allows precision pointing of the much narrower “red” and “blue” communication beams, thereby reducing power consumption while significantly enhancing the signal-to-background ratio and ensuring high speed, accurate communication. Once alignment has been established, the beacon power and angular divergence can be reduced, thereby freeing power to be redirected to the communication lasers.
In addition to establishing alignment between the optics of two satellites or other laser terminals, it is also necessary that the “red” and “blue” optics, i.e. the transmit and receive optics, of each terminal remain aligned with each other, i.e. “co-boresight alignment,” despite any thermal stresses and other factors that might tend to cause a relative misalignment. Periodic calibration and adjustment of this mutual alignment of the transmit/receive optics generally requires the cooperation of a remote terminal as a calibration source. As an alternative, it can be necessary to include additional apparatus in the terminal to enable implementation of a terminal self-contained transmit/receive boresight alignment function.
Furthermore, the fixed configuration of each terminal as being either a “red” terminal or a “blue” terminal limits the flexibility to reconfigure the laser communication network in response to any disruption, for example if one of the terminals should fail due to a malfunction or a malicious attack.
What is needed, therefore, is a laser communication terminal architecture and method with increased flexibility for reconfiguration of the terminals within a laser communication network. Preferably, the improved architecture and method should also enable mutual alignment of the transmit and receive optics of the terminal without requiring the cooperation of a second terminal as a calibration reference.